The present invention generally relates to the field of portable electronic devices, and more particularly to a method and apparatus for attenuating RF emission levels in a portable electronic device.
Advents in the performance of microcomputer based electronics have resulted in dramatic increases in operating speeds of logic switching circuits. Increased switching and operating speeds correspond to increased bandwidths of the electronic signals transmitted within the interior of the electronic device which become a significant source of radio frequency emissions. Unintentional radio frequency emissions undesirably interfere with the internal circuitry of the device itself and with other electronic devices operating within the vicinity of the emitting device. The radio frequency emissions at these higher frequencies may cause undesirable electromagnetic coupling between data paths resulting in cross-channel interference, for example.
The amount of internally generated radio frequency emissions must be limited to the guidelines and regulations set by governmental agencies such as the FCC. For example, the FCC mandates that Class B digital devices operating at frequencies in the range of 30 to 80 MHz may not produce unintentional radiated emissions having a field strength above 100 microvolts per meter at a distance of three meters from the device. For frequencies ranging from 88 to 216 MHz the limit is 150 microvolts per meter.
Shielding, the intentional blocking of an electromagnetic field, of the electronic device is typically utilized to attenuate radio frequency emissions in order to comply with agency regulations. Shielding comprises applying a continuous metallic conductor coating to the interior surfaces of the housing shell of an electronic device to form a gaussian surface which prevents electromagnetic fields from passing through the shielding surface. The ideal material for shielding electromagnetic radiation exhibits both high electrical and magnetic conductivity. The conductor surface acts as an electromagnetic barrier to block unintentional or incidental radio frequency emissions from passing therethrough.
However, no shielding barrier is 100 percent effective in blocking radio frequency emissions. The percent of energy that is blocked is called the shielding effectiveness. Shield continuity, the effectiveness of shielding, is a measure of the degree to which a shield confines or inhibits the electromagnetic field. Because this parameter depends upon the frequency of the electric field, shielding becomes less effective with increasing frequencies. Therefore the reduction of shield continuity at higher frequencies increases in importance as the operational frequencies utilized in portable electronic devices increase.
The effectiveness of an electromagnetic shield is additionally a function of the continuity, or physical completeness, of the barrier. The shield continuity factor is defined as the ratio of the actual shield conductor surface area to the total surface area which the shield area encloses. A solid metal enclosure, with absolutely no holes or gaps and being excellent conductor provides 100 percent shielding continuity. If there are holes or gaps within the shielding, the effectiveness will be less than 100 percent. If it were possible to build an electronic device with a continuous, uninterrupted conductive shield, clearly no charge could enter or leave the terminal. Unfortunately, practical electronic devices have electrical input and output paths and openings required for displays, keys and the like. The required openings in the electronic device prevent the shield from forming a completely closed surface. Therefore, the shielding continuity factor will always be less than one (i.e. less than 100 percent shielding effectiveness).
A possible solution to this problem is to provide specialized treatment of the signal at the point of entry and egress in the device in an attempt to better approximate an ideal, closed surface shield. However, such treatment is often either too costly or too impractical to implement in the constrained space of a small electronic device. Additionally, the shielding coating applied to the internal surfaces of the device increases the cost of the device and requires additional steps in the manufacturing process.
Thus, it would be desirable to reduce the radio frequency emissions from a portable electronic device without the need for shielding the interior surfaces of the housing of the electronic device with an electrically conductive, paramagnetic material. Alternatively, it would be desirable to reduce the radio frequency emissions from a portable electronic device by complementing the less than ideal effectiveness of standard shielding techniques including overcoming the reduction in shielding effectiveness exhibited at higher frequencies.
In addition, it is often desirable to be able to provide a portable electronic device utilizing radio frequency communications signals of increased or greater power without exceeding the maximum signal levels mandated by regulatory agencies.
Accordingly, it is a goal of this invention to provide a method and apparatus for reducing radio frequency emission levels in a portable electronic device.
Another goal is to provide a method and apparatus for reducing radio frequency emission levels without requiring to coat the interior surfaces of the housing of the electronic device with an electrically conductive or paramagnetic material.
A further goal is to provide a method and apparatus for reducing radio frequency emission levels in a portable electronic device which complements the less than ideal shielding effectiveness of an interior shielding coating.
Yet another goal is to provide a method and apparatus for reducing radio frequency emission levels in a portable electronic device which overcomes the loss of shielding effectiveness of an interior shielding coating with increasing frequencies.
These and other goals may be achieved by modulating signals which may be a source for radio frequency emission levels above levels mandated by agency regulations such that the voltage gain levels of the signals are reduced without decreasing the power level of the signals.
More specifically, a method for reducing radio frequency emissions in a portable electronic device comprises the steps of modulating a source signal about its center frequency by modulation of the source frequency with a modulating frequency.
In addition, an apparatus for reducing radio frequency emissions in a portable electronic device comprises a frequency modulator receiving a source signal and modulating signal for modulating the source signal with the modulating signal to produce an output signal which is an attenuated version of the source signal having the same power level as the source signal.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed.